The invention generally relates microchips and, more particularly, the invention relates packaging techniques for microchips
Microelectromechanical systems (xe2x80x9cMEMSxe2x80x9d) are used in a growing number of applications. For example, MEMS currently are implemented as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have a structure suspended above a substrate, and associated electronics that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).
The associated electronics, substrate, and movable structure typically are formed on one or more dies (referred to herein simply as a xe2x80x9cdiexe2x80x9d) that are secured within a package. For example, the package, which typically hermetically seals the die, may be produced from ceramic or plastic. The package includes interconnects that permit the electronics to transmit the movement data to the external devices. To secure the die to the package interior, the bottom surface of the die commonly is bonded (e.g., with an adhesive or solder) to an internal surface (e.g., a die attach pad) of the package. Accordingly, substantially all of the area of the bottom die surface is bonded to the internal surface the package.
Problems can arise, however, when the temperatures of the two surfaces change. In particular, because both surfaces typically have different coefficients of thermal expansion, the package can apply a mechanical stress to the substrate of the die. This stress (referred to in the art as xe2x80x9clinear stress,xe2x80x9d which, in this case, is thermally induced) undesirably can bend or flex the substrate to an unknown curvature. Substrate bending or flexing consequently can affect movement of the die structures and the functioning of the electronics, thus causing the output data representing the property being measured (e.g., acceleration) to be erroneous. In a similar manner, mechanically induced linear or torsional stress applied to the package also can be translated to the die, thus causing the same undesirable effects.
In accordance with one aspect of the invention, a packaged microchip has a stress sensitive microchip having a microchip coefficient of thermal expansion, a package having a package coefficient of thermal expansion, and an isolator having an isolator coefficient of thermal expansion. The isolator is connected between the stress sensitive microchip and the package. In accordance with illustrative embodiments, the microchip coefficient of thermal expansion is closer to the isolator coefficient of thermal expansion than it is to the package coefficient of thermal expansion.
In some embodiments, the difference between the microchip coefficient of thermal expansion and the isolator coefficient of thermal expansion is smaller than the difference between the microchip coefficient of thermal expansion and the package coefficient of thermal expansion. Moreover, the stress sensitive microchip may be a MEMS device, where the stress sensitive microchip includes a movable structure suspended above a substrate having a substrate coefficient of thermal expansion. In such case, the microchip coefficient of thermal expansion may be a function of the substrate coefficient of thermal expansion.
The stress sensitive microchip illustratively has a microchip surface, while the isolator also has an isolator surface. The isolator surface may both face and be connected to the microchip surface. The isolator surface illustratively is connected to no more than a portion of the microchip surface. In other embodiments, the package has a package surface, and a portion of the package surface is connected the isolator. The package surface area is greater than the isolator surface area. Among other materials, the package comprises a ceramic material. The packaged microchip also may include a stress absorbing material that secures the isolator to the package.
In accordance with another aspect of the invention, a sensor has a die comprising a movable structure suspended from a substrate having a substrate coefficient of thermal expansion, an isolator having an isolator coefficient of thermal expansion, and a package forming an inner cavity that contains the die. The package also has a package coefficient of thermal expansion The isolator is connected between the substrate and the package. The substrate coefficient of thermal expansion illustratively is closer to the isolator coefficient of thermal expansion than it is to the package coefficient of thermal expansion.
In some embodiments, the sensor also has a lid that is secured to the package. The lid encloses the inner cavity. The sensor also may include a stress absorbing material that secures the isolator to the package. Moreover, the inner cavity of the package may have a cavity surface, while the substrate may have a substrate surface. The isolator thus forms a space between a portion of the substrate surface and a portion of the cavity surface. Among other things, the die may be one of a gyroscope, an accelerometer, or other type of MEMS device.